1) GIVING MONEY FOR KIDUSHIN WHICH WILL NOT TAKE EFFECT UNTIL AFTER THIRTY DAYS
QUESTION: The Mishnah (58b) teaches that when a man is Mekadesh a woman by giving her money and stipulating that she will become Mekudeshes to him after thirty days ("Harei At Mekudeshes Li l'Achar Sheloshim Yom"), and in the interim another man comes and is Mekadesh her, she is Mekudeshes to the second man. The Gemara asks what her status is if no other man is Mekadesh her within thirty days. Rav and Shmuel answer that she is Mekudeshes (when thirty days pass) even though the money the man gave her for Kidushin is no longer present (it has been spent).
The Gemara explains that this case is not similar to a case of "Mekadesh b'Milvah," in which a man lends money to a woman and then is Mekadesh her with that loan. The Gemara differentiates between a case of "Mekadesh b'Milvah" and the case of the Mishnah
In the case of "Mekadesh b'Milvah," the money was given to the woman specifically as a loan, for her personal use. In the case of the Mishnah, the money was given to the woman for the purpose of Kidushin.
Why does the purpose for which the money originally was given make a difference? The reason why the Kidushin is not valid when one is Mekadesh a woman with a loan is that the act of giving money was performed prior to the time of the Kidushin, and at the time of the Kidushin nothing additional was given to her (see Rashi). The same reasoning applies in every case in which the money was given prior to the time the Kidushin takes effect and the money is no longer present. Although the man is Mekadesh her with the money of the debt and the debt does exist, that debt was already in the domain of the woman prior to the time of the Kidushin and thus it does not qualify as Kesef Kidushin.
ANSWERS:
(a) The RAN explains that since the money originally was given for the purpose of Kidushin, at the time she received the money the woman was Makneh herself to the man. Consequently, the Kidushin took effect at the moment the money was given. In contrast, when the money originally was given for the purpose of a loan, the woman had no intent to become married (and thus the money was given before the time of the Kidushin).
This difference may be understood as follows. The problem of Kidushin with a loan is not that the loan has no monetary value, but rather that the woman is already in possession of that value, as Rashi mentions. In contrast, when the act of Kidushin was performed already (when the money originally was given for the sake of Kidushin and the woman was Makneh herself at that time, even though the Kidushin did not take effect at that time), the pardoning of the debt now may used to complete the Kidushin (since the debt is the present form of the money that was given originally).
(b) The RASHBA has a completely different understanding of the Sugya. The Rashba maintains that if the Kidushin would be made with the money which the man gave to the woman originally (for the sake of Kidushin thirty days later), it indeed would not be valid and would be no different from any other case of "Mekadesh b'Milvah." The reason why the Kidushin is valid in the case of the Mishnah is that the Kidushin is performed with the pleasure the woman receives when the man tells her that she does not need to return the money to him ("Hana'as Mechilah"), rather than with the debt itself. The difference between these two ways of using a loan for Kidushin is stated explicitly by the Gemara earlier (6b).
The RASHBA maintains that in the case of the Mishnah, the money the man gives is automatically viewed as money given for the sake of Kidushin even if he does not state so specifically, for a simple reason: A regular loan becomes the absolute property of the borrower, with the condition that he must pay back its value when the term of the loan ends. When the man gives money to the woman and says, "Harei At Mekudeshes Li l'Achar Sheloshim Yom," the money obviously is not intended to be a loan. If the woman does not wish to become Mekudeshes afterwards, she must give the money back immediately. The money is hers to keep only if she wishes to become Mekudeshes with it. Accordingly, the status of the act of giving the money ("Nesinah") is contingent on whether the woman may keep it. This is not a situation of a debt but rather a situation of deciding whether the money is hers or not. In contrast, in the case of "Mekadesh b'Milvah" the pardoning of the debt is not a valid Kidushin because the money belonged to her before the Kidushin was to take effect. "Mekadesh b'Milvah" is not similar to a case in which the man gives the money to the woman now (and does not require that she return it), which is a valid way to make Kidushin take effect through the Hana'ah of receiving (being allowed to keep) the money. (The fact that she does not have to return the money is considered "Hana'as Mechilah," and thus the Mechilah functions like an ordinary Nesinah.)

59b----------------------------------------59b

2) "ME'ACHSHAV UL'ACHAR SHELOSHIM YOM"
QUESTION: According to Rav Yehudah, Rav and Shmuel disagree about the second case of the Mishnah (58b). In that case, a man says to a woman, "You are hereby Mekudeshes to me from now after thirty days," and within those thirty days a second man comes and is Mekadesh the woman. The Mishnah states that "she is Mekudeshes and she is not Mekudeshes." Rav explains that her status remains in doubt forever, and she needs a Get from both men. Shmuel explains that her status is in doubt only until thirty days have passed. When thirty days have passed, she becomes Mekudeshes to the first man retroactively (if he did not die or retract his intent to marry her), and the Kidushin of the second man is meaningless.
The Gemara explains that Rav and Shmuel disagree about the meaning of the words the man said when he was Mekadesh the woman. Rav maintains that there is a doubt about what the man meant when he said the words "after thirty days." Perhaps he meant to be Mekadesh the woman from this moment but on condition that he not retract his intent for the next thirty days. If that is what he meant, when thirty days pass she becomes married to him retroactively and the Kidushin of the second man is meaningless. On the other hand, perhaps the man meant to retract his statement of "from now" ("me'Achshav") and replace it with "after thirty days" ("l'Achar Sheloshim Yom") so that the Kidushin should take effect only when thirty days pass, in which case she becomes Mekudeshes to the second man who gave her Kidushin within those thirty days.
If the man meant to add a condition to his act of Kidushin when he said "from now after thirty days," what exactly is that condition? The Rishonim give various explanations. RASHI (DH Mesafka Lei) explains that he meant, "You will become Mekudeshes to me from right now (me'Achshav) if I do not change my mind within the next thirty days." The condition is his change of mind.
The RAMBAN explains that the man meant, "You will become Mekudeshes to me from right now (me'Achshav) if we both live another thirty days." The condition is that the Kidushin will take effect only if neither of them dies between now and thirty days.
In his explanation of Shmuel's opinion, Rashi (DH u'Shmuel Amar) writes that the doubt about her status exists only until the end of the thirty-day period; the doubt is that "perhaps the first man will die." Rashi clearly states that if the first man dies within the thirty-day period, his Kidushin does not take effect. This statement, however, seems to contradict Rashi's own explanation of the condition of "me'Achshav ul'Achar Sheloshim Yom," and it seems to follow the Ramban's explanation! According to the Ramban, the condition requires both the man and the woman to be alive at the end of thirty days; the death of either of them will invalidate the Kidushin. According to Rashi, the Kidushin depends on whether or not the Mekadesh changes his mind. If he dies -- but does not change his mind -- within the thirty days, why should the Kidushin not take effect retroactively? (Consequently, the woman retroactively should be married to the first man at the time the second man was Mekadesh her, and thus the second man's Kidushin was meaningless, even if the first man died within thirty days.) (See AVNEI MILU'IM 40:7.)
ANSWER: The AYELES HA'SHACHAR cites the explanation of the RASHBA who explains the words of Rashi. He writes that when the first man says, "You are Mekudeshes to me me'Achshav ul'Achar Sheloshim Yom," he makes the Kidushin dependent upon the presence of his ability to change his mind within thirty days. (That is, it is as if he says, "You will be Mekudeshes to me from now, if for the next thirty days I have the ability to change my mind but I do not change my mind.") The Kidushin does not depend on his change of mind per se, but on his ability and power to change his mind if he wants. Accordingly, when his death occurs within thirty days, although he made no outright change of mind, the ability for him to change his mind throughout the entire thirty-day period no longer exists. Consequently, his condition has not been fulfilled and the Kidushin does not take effect. (See an alternative approach in AVNEI MILU'IM, loc. cit.)